Add discardable memory emulation for non-android/mac platforms

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index d6e91b979d5a46fdd50e6ef07ab4f52a904bc042..5145373486915ae119f7b2d42f31b2f31b8b4830 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -8,9 +8,17 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#if !defined(DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY)
+#include "base/memory/scoped_ptr.h"
+#endif
+#include "base/threading/non_thread_safe.h"
 
 namespace base {
 
+namespace internal {
+class DiscardableMemoryProvider;
+}
+
 enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_FAILED = -1,
   DISCARDABLE_MEMORY_PURGED = 0,
@@ -37,6 +45,9 @@ enum LockDiscardableMemoryStatus {
 //   - Because of memory alignment, the amount of memory allocated can be
 //     larger than the requested memory size. It is not very efficient for
 //     small allocations.
+//   - A discardable memory instance is not thread safe. It is the
+//     responsibility of users of discardable memory to ensure there are no
+//     races.
 //
 // References:
 //   - Linux: http://lwn.net/Articles/452035/
@@ -51,8 +62,9 @@ class BASE_EXPORT DiscardableMemory {
   // The opened file will also be closed after this.
   ~DiscardableMemory();
 
-  // Check whether the system supports discardable memory.
-  static bool Supported();
+  // Check whether the system supports discardable memory natively. Returns
+  // false if the support is emulated.
+  static bool SupportedNatively();
 
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
@@ -87,6 +99,10 @@ class BASE_EXPORT DiscardableMemory {
   static void PurgeForTesting();
 
  private:
+#if !defined(DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY)
+  scoped_ptr<uint8, FreeDeleter> acquired_memory_;
+#endif
+
 #if defined(OS_ANDROID)
   // Maps the discardable memory into the caller's address space.
   // Returns true on success, false otherwise.